Fix sentinel_kwargs load from ENV

[vaaalik]

Celery provider is expecting sentinel_kwargs to be a dict, current implementation is only working when config is set through airflow.cfg.
When the env variable AIRFLOW__CELERY_BROKER_TRANSPORT_OPTIONS__SENTINEL_KWARGS is set, it loads it as a string and thus fails. This pull request fixes this issue and both airflow.cfg and env variable will work. (Also removed additional copy of broker_transport_options dict since it serves no purpose)

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[boring-cyborg]

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[hussein-awala]

I think you can fix the issue with this small change:

-        sentinel_kwargs = broker_transport_options.get("sentinel_kwargs")
+        sentinel_kwargs = json.loads(broker_transport_options.get("sentinel_kwargs"))

Also you need to add a new test for this change.

[vaaalik]

Added test to check that config is loaded from a string.

[boring-cyborg]

Awesome work, congrats on your first merged pull request! You are invited to check our Issue Tracker for additional contributions.